Antenna for a plurality of bands

ABSTRACT

The present invention provides an antenna for multiple bands employing a single antenna element  10 , capable of operating in multiple frequency bands, and ideal for size reduction purposes. One end A of an antenna element  10  is electrically connected to a feeding point  12  and intermediate points B and C and the other end thereof is electrically connected via switches SWb, SWc, and SWd to a ground conductor  14 . The electrical lengths of the antenna element  10  from the terminal to the intermediate points B and C plus connection lines from these points via the switches SWb and SWc to the ground conductor 14 and the electrical length from the one end A to the other end D plus a connection line from the other end via the switch SWd to the ground conductor  14  are set to be capable of resonating different desired frequency bands. By closing one of the switches SWb, SWc, and SWd, one of the desired frequencies can be selected and the antenna can resonate with that frequency. Thus, the antenna employing the single antenna element  10  can operate in multiple frequency bands.

TECHNICAL FIELD

The present invention relates to an antenna for multiple bands,employing a single antenna element adapted so it can operate in multiplefrequency bands.

BACKGROUND ART

Recent mobile communication has developed rapidly. Among others, mobilephones have proliferated outstandingly and improvements have been madeto reduce their size and weight significantly. According to mobile phonestandards, two particular frequency bands are used respectively indifferent regions: in Japan, a 800 MHz band and a 1.5 GHz band forPersonal Digital Cellular (PDC); in Europe, a 900 MHz band and a 1.9 GHzband for Global System for Mobile Communications (GSM); and in U.S., a800 MHz band for Advanced Mobile Phone System (AMPS) and a 1.9 GHz bandfor Personal Communications System (PCS). Moreover, communicationsystems such as Global Positioning System (GPS) using 1.5 GHz, Bluetoothusing a 2.4 GHz band, and International Mobile Telecommunications (IMT)2000 using a 2 GHz band are put in practical use for mobilecommunication and data transmission. If a single antenna is capable ofoperating in the above-mentioned multiple frequency bands, it would beideal for the purpose of reducing antenna size and weight.

Furthermore, there is a plan in progress to adopt the GSM that has beenused in Europe in U.S. as a mobile phone scheme so that a same mobilephone can be used in U.S. and Europe. However, the GSM in Europe uses aband of 880 to 960 MHz and a band of 1710 to 1880 MHz, whereas the GSMin U.S. is designed to use a band of 824 to 894 MHz and a band of 1850to 1990 MHz. An antenna capable of operating in the frequency bands inboth Europe and U.S. is required to cover both a wide frequency band of136 MHz ranging from 824 to 960 MHz and a wide frequency band of 280 MHzranging from 1710 to 1990 MHz.

So far, a single antenna capable of operating in the above multiplefrequency bands has not existed. So far, an antenna covering the widefrequency bands so it can operate in the GSM frequency bands in bothU.S. and Europe has not existed.

By the way, antennas with reduced size and weight for use in mobilephones have been proposed in Japanese Patent Application Laid-Open(JP-A) No. 2001-284935 and Japanese Patent Application Laid-Open (JP-A)No. 2002-43826. The principles of these techniques will be brieflydescribed below. FIG. 26 shows a basic structure of an antenna of priorart, wherein one end of an antenna element 10 is connected to a feedingpoint 12 and the other end thereof is electrically connected to a groundconductor 14. The most part of the antenna element 10 is straightened inapproximately parallel with the ground conductor 14 except the uprightends for the connections to the feeding point 12 and the groundconductor 14. The entire electrical length of the antenna element 10 isset to ½ wavelength (λ/2) or 1 wavelength (λ) of a frequency band inwhich the antenna operates. Moreover, the antenna element maybe formedin a coil or meandering pattern or appropriately bent into a loop forsize reduction purposes. These techniques can be used for only a singlefrequency band. In FIG. 26, a dotted line denotes current distribution.

FIG. 27 shows another prior art antenna, wherein a capacitor 16 isinserted in series in the center of the antenna element 10 of prior artshown in FIG. 26. The electrical length of the antenna element plus thecapacitor 16 is set to ½ wavelength of a frequency band in which theantenna operates. Current distribution denoted by a dotted line in FIG.27 indicates that an in-phase current is produced in the antenna element10 and this is effective particularly for a case where antennadirectivity is important.

FIG. 28 shows yet another prior art antenna, wherein the capacitor 16 isinserted at a point on the antenna element 10, nearer to the feedingpoint 12, not in the center, as a modification to the prior art antennashown in FIG. 27. FIG. 29 shows yet another prior art antenna, whereintwo parallel conductors 28 which are disconnected in direct current areinserted in series between the ends of the antenna element 10. The twoparallel conductors 18 are inductively coupled together and function asa single antenna element as a whole.

FIG. 30 shows a further prior art antenna, wherein a matching circuit 20is inserted between one end of the antenna element 10 and the feedingpoint and the other end of the antenna element 10 is electricallyconnected to the ground conductor 14. In the prior art antenna shown inFIG. 30, the length of the antenna element 10 is not required to be ½wavelength of a frequency band in which the antenna operates. Theantenna element 10 and the matching circuit 20 should be setappropriately so that the electrical length containing the antennaelement 10 and the matching circuit 20 will be ½ wavelength.

However, any antenna of the above prior art is designed to operate in asingle frequency band and cannot operate in multiple frequency bands.Thus, a mobile phone that uses two frequency bands needs two antennasfor different frequency bands. A mobile communication device in which aplurality of communication systems including GPS are installed needs aplurality of antennas. Hence, it is difficult to reduce the size andweight of a mobile communication device by using any of the above priorart antennas.

It is therefore an object of the present invention, which has been madein view of the above circumstances of prior art, to provide an antennafor multiple bands employing an single antenna element 10, the antennabeing capable of operating in multiple frequency bands and ideal forsize and weight reduction purposes.

DISCLOSURE OF THE INVENTION

An antenna for multiple bands of the present invention is configuredsuch that one end of an antenna element is electrically connected to afeeding point and the other end thereof is electrically connected to aground conductor, at least one intermediate point and the other end ofthe antenna element are electrically connected via switches,respectively, to the ground conductor, the electrical length of theantenna element from the feeding point to the other end plus aconnection line from the other end via one switch to the groundconductor and the electrical length of the antenna element from thefeeding point to the at least one intermediate point plus a connectionline from the at least one intermediate point via another switch to theground conductor are set to be capable of resonating different desiredfrequency bands respectively.

By employing a single antenna element and using the switches insertedbetween the intermediate points and the other end of the antenna elementand the ground terminal, a desired number of frequency bands can be set.Thus, this antenna is favorable as a small antenna for mobilecommunication and operation in multiple frequency bands.

An antenna in which one end of an antenna element is electricallyconnected to a feeding point and the other end thereof is electricallyconnected to a ground conductor may be configured such that at least oneintermediate point and the other end of the antenna element areelectrically connected via series resonant circuits, each comprising acapacitor and a coil, respectively, to the ground conductor, theelectrical length of the antenna element from the feeding point to theother end is set to make its resonant frequency equal to a resonantfrequency of one series resonant circuit connected to the other end, theelectrical length of the antenna element from the feeding point to theat least one intermediate point is set to make its resonant frequencyequal to a resonant frequency of another series resonant circuitconnected to the at least one intermediate point, and the resonantfrequencies of the electrical lengths are set to different desiredfrequency bands respectively.

An antenna in which one end of an antenna element is electricallyconnected to a feeding point and the other end thereof is electricallyconnected to a ground conductor can also be configured such that atleast one intermediate point and the other end of the antenna elementare electrically connected via filters, respectively, to the groundconductor, one filter connected to the other end allows passage of aresonant frequency with which the electrical length of the antennaelement from the feeding point to the other end resonates, anotherfilter connected to the at least one intermediate point allows passageof a resonant frequency with which the electrical length of the antennaelement from the feeding point to the at least one intermediate pointresonates, each filter blocks passage of a frequency other than theresonant frequency with which the electrical length to the position towhich the filter is connected resonates, and the resonant frequencies ofthe electrical lengths are set to different desired frequency bandsrespectively.

Furthermore, an antenna in which one end of an antenna element iselectrically connected to a feeding point and the other end thereof iselectrically connected to a ground conductor can also be configured suchthat one intermediate point and the other end of the antenna element areelectrically connected via parallel resonant circuits, each comprising acapacitor and a coil, respectively, to the ground conductor, theelectrical length of the antenna element from the feeding point to theother end is set to make its resonant frequency equal to a resonantfrequency of one parallel resonant circuit connected to the oneintermediate point, the electrical length of the antenna element fromthe feeding point to the one intermediate point is set to make itsresonant frequency equal to a resonant frequency of another parallelresonant circuit connected to the other end, and the resonantfrequencies of the electrical lengths are set to different desiredfrequency bands respectively.

The antenna for multiple bands thus configured employing the singleantenna element is capable of simultaneous antenna operation in multiplefrequency bands. Thus, this antenna is favorable for mobilecommunications in a situation where simultaneous antenna operation inmultiple frequency bands is required, for instance, both GPS and mobilephone systems are used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a principle structure of a first embodiment of an antennafor multiple bands of the present invention, using switches.

FIG. 2 shows a principle structure of a second embodiment of an antennafor multiple bands of the present invention, using series resonantcircuits.

FIG. 3 shows a principle structure of a third embodiment of an antennafor multiple bands of the present invention, using parallel resonantcircuits.

FIG. 4 shows a principle structure of a fourth embodiment of an antennafor multiple bands of the present invention, using filters.

FIG. 5 shows an antenna structure modification to the first embodiment,wherein a capacitor is inserted in series between the feeding point andone intermediate point nearer to the feeding point on the antennaelement.

FIG. 6 shows another antenna structure modification to the firstembodiment, wherein inductively coupled parallel conductors are insertedin series between the feeding point and one intermediate point nearer tothe feeding point on the antenna element.

FIG. 7 shows yet another antenna structure modification to the firstembodiment, wherein a matching circuit is inserted between one end ofthe antenna element and the feeding point.

FIG. 8 is comprised of FIG. 8A and FIG. 8B, in which FIG. 8A depicts acase where, in the structure of the first embodiment antenna shown inFIG. 1, the electrical length of the antenna element to a point ofconnection of an open switch resonates with a frequency in the vicinityof a frequency band with which the electrical length of the antennaelement to a point of connection of a closed switch resonates; and FIG.8B is a graph to depict an antiresonance point produced by the tworesonant frequencies which are close to each other.

FIG. 9 shows an antenna structure of a fifth embodiment devised to solvethe problem described with the antenna structure shown in FIG. 8.

FIG. 10 shows a sixth embodiment of a concrete construction of thefourth embodiment antenna for multiple bands of the present inventionshown in FIG. 4.

FIG. 11 shows a seventh embodiment of a concrete construction of thefourth embodiment antenna for multiple bands of the present inventionshown in FIG. 4, the seventh embodiment having a dielectric and acapacitance coupled antenna element, wherein FIG. 11A is a plan view ofthe seventh embodiment and FIG. 11B is a front view thereof.

FIG. 12 shows a meandering pattern of the antenna element bent widthwiseat a right angle, so that an “L” shape section is viewed from its endside.

FIG. 13 shows the meandering pattern of the antenna element bentwidthwise at a right angle twice, so that an angular “U” shape sectionis viewed from its end side.

FIG. 14 shows the meandering pattern of the antenna element bentwidthwise at a right angle repeatedly, so that a meandering shapesection is viewed from its end side as well.

FIG. 15 is an outside perspective view of a concrete example of anantenna for multiple bands of the present invention on the assumptionthat the antenna is used in a mobile phone.

FIG. 16 is a structural diagram of the antenna for multiple bands shownin FIG. 15.

FIG. 17 shows a VSWR characteristic graph when SW1 is open and SW2 isclosed in the antenna for multiple bands shown in FIG. 16.

FIG. 18 shows a Smith chart when SW1 is open and SW2 is closed in theantenna for multiple bands shown in FIG. 16.

FIG. 19 shows a VSWR characteristic graph when SW1 is closed and SW2 isopen in the antenna for multiple bands shown in FIG. 16.

FIG. 20 shows a Smith chart when SW1 is closed and SW2 is open in theantenna for multiple bands shown in FIG. 16.

FIG. 21 shows an antenna structure modification to the first embodiment,wherein the other end of the antenna element is electrically connecteddirectly to the ground conductor without intervention of the switch SWd.

FIG. 22 is an outside perspective view of a concrete example of theantenna for multiple bands of the present invention in which the otherend of the antenna element is electrically connected directly to theground conductor, shown in FIG. 21, on the assumption that the antennais used in a mobile phone.

FIG. 23 is an outside perspective view of another concrete example ofthe antenna for multiple bands of the present invention in which theother end of the antenna element is electrically connected directly tothe ground conductor, shown in FIG. 21, on the assumption that theantenna is used in a mobile phone.

FIG. 24 is an outside perspective view of yet another concrete exampleof the antenna for multiple bands of the present invention in which theother end of the antenna element is electrically connected directly tothe ground conductor, shown in FIG. 21, on the assumption that theantenna is used in a mobile phone.

FIG. 25 shows an antenna embodiment in which intermediate points and theother end of the antenna element are electrically connected to theground conductor via different types of electric circuits, a switch, aseries resonant circuit, and a filter.

FIG. 26 shows a basic structure of an antenna of prior art.

FIG. 27 shows another prior art antenna, wherein a capacitor is insertedin series in the center of the antenna element of the antenna of priorart shown in FIG. 26.

FIG. 28 shows yet another prior art antenna, wherein the capacitor isinserted at a point on the antenna element, nearer to the feeding point12, of the antenna of prior art shown in FIG. 26.

FIG. 29 shows yet another prior art antenna, wherein two parallelconductors which are inductively coupled are inserted in series betweenthe ends of the antenna element, nearer to the feeding point, of theantenna of prior art shown in FIG. 26.

FIG. 30 shows a further prior art antenna, wherein a matching circuit isinserted between one end of the antenna element and the feeding point ofthe antenna of prior art shown in FIG. 26.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, a first embodiment of the present inventionwill be described below. FIG. 1 shows a principle structure of a firstembodiment of an antenna for multiple bands of the present invention,using switches. In FIG. 1, one end of the antenna element 10 isconnected to a feeding point 12 and the other end thereof is connectedvia a switch SWd to a ground conductor 14. Two intermediate points ofthe antenna element are connected via switches SWb and SWc,respectively, to the ground conductor 14. The most part of the antennaelement 10 is straightened in approximately parallel with the groundconductor 14 except the upright sections for the connections to thefeeding point 12 and the switches. In the antenna element 10, theelectrical length from a point A (one end of the antenna element 10) ofthe feeding point 12 connection to a point B (one intermediate point onthe antenna element 10) of the switch SWb connection is set to ½wavelength of a first frequency band f1, the electrical length from thepoint A to a point C (the other intermediate point on the antennaelement 10) of the switch SWc connection is set to ½ wavelength of asecond frequency band f2, and the electrical length from the point A toa point D (the other end of the antenna element 10) of the switch SWdconnection is set to ½ wavelength of a third frequency band f3. It isnatural that the center frequencies of the first to third frequencybands f1, f2, and f3 are f3<f2<f1. Of course, the first to thirdfrequencies f1, f2, and f3 are set, respectively, for multiple frequencybands in which the antenna operates.

In the first embodiment of the above-described antenna structure, whenthe switches SWb and SWc are open and only the switch SWd is closed, theantenna with the electrical length from the point A to the point D onthe antenna element 10 is formed and functions as the antenna resonatingwith the third frequency band f3, as is the case for the prior artantenna shown in FIG. 26. Similarly, when the switches SWb and SWd areopen and only the switch SWc is closed, the antenna with the electricallength from the point A to the point C on the antenna element 10 isformed and functions as the antenna resonating with the second frequencyband f2. When the SWc and SWd are open and only the switch SWb isclosed, the antenna functions as the one resonating with the firstfrequency band f1.

As described above, the first embodiment of the antenna for multiplebands of the present embodiment employs the single antenna element 10,which is preferable for size and weight reduction purposes. By providingas many switches SWb, SWc, and SWd as the required number of frequencybands for which the antenna is designed, the single antenna element 10can be made adaptive to two or more frequency bands. The switches SWb,SWc, and SWd in the first embodiment are not limited to mechanical ones;of course, they may be semiconductor switches employing pin diodes orthe like.

With reference to FIG. 2, a second embodiment of the present inventionis now described. FIG. 2 shows a principle structure of a secondembodiment of an antenna for multiple bands of the present invention,using series resonant circuits. In FIG. 2, the difference from FIG. 1lies in that the antenna is provided with first to third series resonantcircuits 22, 24, and 26 instead of the switches SWb, SWc, and SWd. Theresonant frequency of the first series resonant circuit 22 insertedbetween the one intermediate point B on the antenna element 10 and theground conductor 14 is set to the first frequency band f1 with which theelectrical length from the feeding point A to the point B resonates.Similarly, the resonant frequency of the second series resonant circuit24 inserted between the other intermediate point C on the antennaelement 10 and the ground conductor 14 is set to the second frequencyband f2 with which the electrical length from the feeding point A to thepoint C resonates. The resonant frequency of the third series resonantcircuit 26 inserted between the other end D of the antenna element 10and the ground conductor 14 is set to the third frequency band f3 withwhich the electrical length from the feeding point A to the other end Dresonates.

In the second embodiment of the above-described antenna structure, atthe first frequency band f1, the antenna operates with the same actionas the one intermediate point C was electrically short-circuited via thefirst series resonant circuit 22 to the ground conductor 14 andfunctions as the one resonating with the first frequency band f1.Similarly, at the second frequency band f2, the other intermediate pointD is short-circuited via the second series resonant circuit 24 andgrounded and the antenna functions as the one resonating with the secondfrequency band f2. At the third frequency band f3, the other end D isshort-circuited via the second series resonant circuit 24 and groundedand the antenna functions as the one resonating with the secondfrequency band f3. Thus, the antenna of the second embodiment is enabledto operate in the first to third frequency bands f1, f2, and f3 at thesame time and a circuit or equivalent for frequency separation should beprovided appropriately near the feeding point 12. Hence, the antenna formultiple bands of the second embodiment employing the single antennaelement 10 is preferable as an antenna for mobile communications in ansituation where simultaneous antenna operation in multiple bands isrequired, for instance, both GPS and mobile phone systems are used. Inthe above description, the series resonant circuits 22, 24, 26 aredesigned to behave such that those other than one that is electricallyshort-circuited to resonate with a frequency band are electricallydisconnected. It will be appreciated that the electrical lengths of theantenna element 10 from the feeding point A to the intermediate pointsB, C, and the other end D may be set appropriately in consideration ofthe electrical effect of a series resonant circuit, when grounded, onthe remaining non-grounded ones for other frequency bands.

With reference to FIG. 3, a third embodiment of the present invention isnot described. FIG. 3 shows a principle structure of a third embodimentof an antenna for multiple bands of the present invention, usingparallel resonant circuits. In FIG. 3, the difference from FIG. 2 liesin that only a single intermediate point B is present on the antennaelement 10, a first parallel resonant circuit 28 is inserted between theintermediate point B and the ground conductor 14, and a second parallelresonant circuit 30 is inserted between the other end D and the groundconductor 14. The resonant frequency of the first parallel resonantcircuit 28 is set to the third frequency band f3 with which theelectrical length from the feeding point A to the other end D resonatesand the first parallel resonant circuit 28 behaves as a trap circuit ofthe third frequency band f3. The intermediate point B is electricallyshort-circuited to the ground conductor 14 at the first frequency bandf1 with which the electrical length from the point A to the point Bresonates and electrically disconnected from the ground conductor 14 atthe third frequency band f3. This makes the antenna function as the oneresonating with the first frequency band f1. Similarly, the other end Dis electrically disconnected from the ground conductor 14 at the firstfrequency band f1 and electrically short-circuited to the groundconductor 14 at the third frequency band. This makes the antennafunction as the one resonating with the third frequency band f3. In theabove description, the parallel resonant circuits 28 and 30 are designedto behave such that one not involved in a trap of a frequency band doesno electrical action. It will be appreciated that the electrical lengthsof the antenna element 10 from the feeding point A to the intermediatepoint B and the other end D may be set appropriately in consideration ofthe electrical effect of one of the parallel resonant circuits 28 whenit performs a frequency trap on the other for a frequency band nottrapped. Thus, the antenna for multiple bands of the third embodimentemploying the single antenna element 10 is capable of simultaneousantenna operation in multiple bands in a similar manner as the secondembodiment and is preferable as an antenna for mobile communications inan situation where simultaneous antenna operation in multiple bands isrequired, for instance, both GPS and mobile phone systems are used.

In the second and third embodiments, the series and parallel resonantcircuits may be configured as either lumped parameter circuits ordistributed parameter circuits.

With reference to FIG. 4, a fourth embodiment of the present inventionis now described. FIG. 4 shows a principle structure of a fourthembodiment of an antenna for multiple bands of the present invention,using filters. In FIG. 4, the difference from FIG. 1 lies in that theantenna is provided with a high-pass filter 32, a bandpass filter 34,and low-pass filter 36 instead of the switches SWb, SWc, and SWd. Thehigh-pass filter 32 inserted between the one intermediate point B on theantenna element 10 and the ground conductor 14 is set to allow thepassage of the first frequency band f1 with which the electrical lengthfrom the feeding point A to the point B resonates and block the passageof other second and third frequency bands f2 and f3. The bandpass filter34 inserted between the other intermediate point C and the groundconductor 14 is set to allow the passage of the second frequency band f2with which the electrical length from the feeding point A to the point Cresonates and block the passage of other first and third frequency bandsf1 and f3. Similarly, the low-pass filter 36 inserted between the otherend D and the ground conductor 14 is set to allow the passage of thethird frequency band f3 with which the electrical length from thefeeding point A to the other end D resonates and block the passage ofother first and second frequency bands f1 and f2.

In the fourth embodiment of the above-described antenna structure, thefilters 32, 34, and 36 behave to make the ground connection of one ofthe intermediate points B, C, and the other end D at the frequency bandwith which the electrical length from the feeding point A to that pointresonates and disconnect the ground connection at other frequency bands.Thus, the fourth embodiment antenna is capable of simultaneous antennaoperation in the first to third frequency bands f1, f2, and f3 in asimilar manner as the second embodiment. Hence, the antenna for multiplebands of the fourth embodiment employing the single antenna element 10is preferable as an antenna for mobile communications in an situationwhere simultaneous antenna operation in multiple bands is required, forinstance, both GPS and mobile phone systems are used, as is the case forthe second and third embodiments. It will be appreciated that thehigh-pass filter 32 and the low-pass filter 36 may be bandpass filtersallowing the passage of the first frequency band f1 and the thirdfrequency band f3, respectively.

The first embodiment antenna shown in FIG. 1 maybe modified such that acapacitor 16 is inserted in series between the feeding point 12 and oneintermediate point nearer to the feeding point on the antenna element10, as is shown in FIG. 5. A capacitance coupled circuit may be usedinstead of the capacitor 16. The first embodiment antenna shown in FIG.1 may be modified such that two parallel conductors 18 which areinductively coupled together are inserted in series between the feedingpoint 12 and one intermediate point nearer to the feeding point on theantenna element 10, as is shown in FIG. 6. Furthermore, the firstembodiment antenna shown in FIG. 1 may be modified such that a matchingcircuit 20 is inserted between one end A of the antenna element 10 andthe feeding point 12, as is shown in FIG. 7. In the first embodimentmodifications shown in FIGS. 5 through 7, the electrical lengths shouldbe set in consideration of the capacitor 16, parallel conductors 18, andmatching circuit 20 inserted. Furthermore, the antenna structures of thesecond through fourth embodiments may be modified, like the firstembodiment modifications shown in FIGS. 5 through 7. Thereby, theelectrical lengths of the single antenna element 10 enabling the antennato operate in multiple bands can be designed appropriately by provisionof the capacitor C or the matching circuit 20.

By the way, in the first embodiment antenna shown in FIG. 1, assume thatthe switch SWb is closed, while the switches SWc and SWd are open, inthe antenna structure shown in FIG. 8A, and the electrical length of theantenna element 10 from the feeding point A to the point B resonateswith the first frequency band f1. At this time, if the electrical lengthof the antenna element 10 from the feeding point A to the point C and/orthe electrical length from the feeding point A to the other end D withregard to the wavelength (λ) of the first frequency f1 are contingentlyλ·(¼+n·½)±Δ (where n is an integer), such as, for example, λ 5/4±Δ, asindicated by a dotted line, that length will also resonate with afrequency f1±α in the vicinity of the first frequency f1. Inconsequence, there is a possibility that an antiresonance point isproduced by the first frequency band f1 and the frequency f1±α in thevicinity of the first frequency, as is shown in FIG. 8B. Thisantiresonance point deteriorates a VSWR characteristic and results in adecrease in the antenna gain. In view hereof, it is desirable that anantiresonance point does not exist within a frequency bandwidth to beused.

A fifth embodiment of an antenna structure which is shown in FIG. 9 isan example of means for solving this problem. In the fifth embodiment,one end of the switch SWb is connected to the one intermediate point Bon the antenna element 10 and the other end of the swith SWb isconnected to the ground connector 14 directly. One end of the switch SWcis connected to the other intermediate point C on the antenna element 10and the other end of the switch SWc is connected via an extension coil Linserted in series to the ground conductor 14. One end of the switch SWdis connected to the other end D and the other end of the switch SWd isconnected via a short capacitor C inserted in series to the groundconductor 14. By inserting the extension coil L appropriately, it ispossible to shorten the electrical length of the antenna element 10 fromthe feeding point A to the other intermediate point C. By inserting theshort capacitor C appropriately, it is possible to elongate theelectrical length from the feeding point A to the other end D on theantenna element 10. Thereby, it can be avoided during the firstfrequency band H operation that the electrical lengths from the feedingpoint to the point C and the other end D resonate with a frequency inthe vicinity of the first frequency band f1, resulting in anantiresonance point within the frequency bandwidth in use. Here,needless to say, with the switch SWc closed, the electrical length ofthe antenna element 10 from the feeding point A to the intermediatepoint C modified by the extension coil L is set to ½ wavelength of thesecond frequency band f2. With the switch SWd closed, the electricallength of the antenna element 10 from the feeding point 10 to the otherend point D modified by the short capacitor C is set to ½ wavelength ofthe third frequency band f3.

While the possibility that, when the electrical length from the feedingpoint to the one intermediate point B resonates with the first frequencyf1, the electrical lengths from the feeding point to the intermediatepoint C and the other end D resonate with a frequency in the vicinity ofthe first frequency has been illustrated in the antenna structure shownin FIG. 8, there is also a possibility that, when the electrical lengthfrom the feeding point to the other intermediate point C resonates withthe second frequency band 12, the electrical length from the feedingpoint to the other end D resonates with a frequency in the vicinity ofthe second frequency. In such cases, it will easily be appreciated thatthe intermediate points B, C, and the other end D should be connected,respectively, to one ends of the switches SWb, SWc, and SWd, and in theother ends of others of these switches SWb, SWc and Swd should beconnected to the ground conductor 14 directly, and the others of theseswitches SWb, SWc, and SWd should be connected, respectively, to theground conductor 14 appropriately with an extension coil or a shortcapacitor inserted in series therebetween, to prevent an antiresonancepoint from being within any frequency bandwidth in use.

Next, concrete configuration examples of the antenna for multiple bandsof the present invention will be described. FIG. 10 shows a sixthembodiment of a concrete construction of the fourth embodiment antennafor multiple bands of the present invention shown in FIG. 4. In FIG. 10,the antenna element 10 is formed along an imaginary circular cylinderplane in a meandering pattern turned around repeatedly between both endsof the cylinder, parallel to the center axis of the cylinder, for sizereduction purposes. The antenna element is sheathed in a cover 40 madeof suitable insulating resin. One end A, the intermediate points C, D,and the other end D of the antenna element 10 are appropriately drawnout and electrically connected to connection terminals not shown. On theother hand, the feeding point 12, the high-pass filter 32, bandpassfilter 34, and the low-pass filter 36 are provided on a substrate 42 andelectrically connected to connection terminals appropriately. On thesubstrate 42, a ground conductor not shown is provided and the filters32, 34, and 36 are grounded to it. The substrate 42 is housed in acasing not shown. In the casing, the antenna element 10 is installed ina position so as to protrude outside and to be removable and the one endA, the intermediate points B, C, and the other end D of the antennaelement 10 are positioned so that they can be connected to anddisconnected from the feeding point and the filters 32, 34, and 36,respectively. Of course, the antenna element 10 shown in FIG. 10 can beapplied to the first to third embodiments shown in FIGS. 1 through 3,respectively. By forming the antenna element 10 in a meandering pattern,the outside dimension of the whole antenna element 10 can be reduced.Because the antenna element 10 is formed in the meandering pattern whichis formed along the imaginary circular cylindrical plane and itsexternal connections can be connected to and disconnected from itsassociated component circuits, only the antenna element 10 can beinstalled later in the antenna manufacturing process. If the antennafails, it can be replaced with ease. This antenna embodiment ispreferable as an antenna that is installed protruding outside the mobilephone casing.

FIG. 11 shows a seventh embodiment of a concrete construction of thefourth embodiment antenna for multiple bands of the present inventionshown in FIG. 4, the seventh embodiment having a dielectric and acapacitance coupled antenna element, wherein FIG. 11A is a plan view ofthe seventh embodiment and FIG. 11B is a front view thereof. In FIG. 11,the antenna element 10, the feeding point A, and the filters 32, 34, and36 a are arranged on the surfaces of the dielectric 44. The antennaelement 10 is configured to be separated into two parts by a gap in anintermediate position nearer to the feeding point, so that the ends ofthe two parts facing each other across the gap are capacitance coupled38 together. The antenna element can be formed in a thin metal film onthe surfaces of the dielectric 44 by plating, vapor deposition, and thelike, which is preferable for mass production. Because the dielectric 44has an effect of decreasing wavelength, the physical length of theantenna element 10 can be shortened and, accordingly, this embodiment ispreferable for size reduction. Although the antenna element 10 is formedon the surfaces of the dielectric 44, the dielectric 44 may be layeredand the filters 32, 34, and 36 may be placed between layers in thedielectric 44. The filters 32, 34, and 36 may be placed in any positionin the dielectric 44.

To further reduce the dimensions of the antenna element 10, a meanderingpattern of the antenna element on the flat may be bent widthwise at aright angle, so that an “L” shape section is viewed from its end side,as an example which is shown in FIG. 12. As another example which isshown in FIG. 13, the meandering pattern of the antenna element may bebent widthwise at a right angle twice, so that an angular “U” shapesection is viewed from its end side. As yet another example which isshown in FIG. 14, the meandering pattern of the antenna element may bebent widthwise at a right angle repeatedly, so that a meandering shapesection is viewed from its end side as well.

Moreover, an eighth embodiment of the present invention will bedescribed with reference to FIGS. 15 through 20

FIG. 15 is an outside perspective view of a concrete example of anantenna for multiple bands of the present invention on the assumptionthat the antenna is used in a mobile phone. FIG. 16 is a structuraldiagram of the antenna for multiple bands shown in FIG. 15. FIG. 17shows a VSWR (voltage standing wave ratio) characteristic graph when SW1is open and SW2 is closed in the antenna for multiple bands shown inFIG. 16. FIG. 18 shows a Smith chart when SW1 is open and SW2 is closedin the antenna for multiple bands shown in FIG. 16. FIG. 19 shows a VSWRcharacteristic graph when SW1 is closed and SW2 is open in the antennafor multiple bands shown in FIG. 16. FIG. 20 shows a Smith chart whenSW1 is closed and SW2 is open in the antenna for multiple bands shown inFIG. 16.

In FIG. 15, the ground conductor 14 is a rectangle with a short side of40 mm and a long side of 100 mm and the antenna element 10 is formed,bordering on one short side of the ground conductor, separated from theground conductor 14. This antenna element 10 is formed in an meanderingpattern turned around repeatedly in a direction parallel to the longsides of the rectangular ground conductor 14 and the meandering patternis bent widthwise at a right angle so that a substantially “L” shapesection is viewed from its end side. One end A, an intermediate point B,and the other end D of the antenna element 10 are connectedappropriately to associated circuits mounted on a substrate 4 on whichthe ground conductor 14 is provided, without being electricallyconnected to the ground conductor 14. As shown in FIG. 16, the one end Ais connected via a matching circuit 20 to the feeding point 12, theintermediate point B is connected via a first switch SW1 to the groundconductor 14, and the other end D is grounded via a second switch SW2.The antenna embodiment shown in FIGS. 15 and 16 is configured to becapable of operating in two frequency bands for mobile phone use, an 800MHz band and a 1800 MHz band.

When the first switch SW1 is open and the second switch SW2 is closed,the antenna element resonates with a low frequency band and a good VSWRcharacteristic of less than 2 is measured in a range of 824-960 MHzaccording to FIG. 17. Also, impedance near to approximately 50Ω isobtained in the range of 824-960 MHz, as indicated in FIG. 18. Thus,this antenna embodiment can be used as an antenna operating over a widefrequency band covering both an 824-894 MHz GSM band to be applied inU.S. and an 880-960 MHz GSM band applied in Europe. When the firstswitch SW1 is closed and the second switch SW2 is open, the antennaelement resonates with a high frequency band and a good VSWRcharacteristic of less than 2.6 is measured in a range of 1710-1990 MHzaccording to FIG. 19. Also, impedance near to approximately 50 Ω isobtained in the range of 1710-1990 MHz, as indicated in FIG. 20. Thus,this antenna embodiment can be used as an antenna operating over a widefrequency band covering both an 1850-1990 MHz GSM band to be applied inU.S. and a 1710-1880 MHz GSM band applied in Europe. Because the antennaelement 10 is formed, bordering on the one short side of the rectangularground conductor 14, this antenna embodiment is preferable for a mobilephone construction with folding halves (shells) in which the groundconductor 1 is provided in an operation side shell with operationbuttons arranged thereon and the antenna element 10 is installed nearthe folding hinges. This antenna embodiment is also preferable for amobile phone construction in which the antenna element 10 is installedon the end (the moving end opposite to the end with the hinges) ofeither the operation side shell or a display side shell having a displayscreen.

The above antenna embodiments shown in the FIGS. 1, 2, and 4 through 11are designed to be capable of operating in three frequency bands and theantenna embodiments shown FIGS. 3, 15, and 26 are designed to be capableof operating in two frequency bands; however, the number of frequencybands may be set appropriately so that the antenna can cover therequired number of frequency bands for which the antenna is designed.Size reduction of the antenna for multiple bands of the presentinvention by forming the antenna element 10 in a meandering pattern orby other ways and the dimensions and shape of the ground conductor 14have an influence on the antenna characteristics. If, for example, thedimensions of the ground conductor 14 shown in FIG. 15 are modified to arectangle with a short side of 40 mm and a long side of 80 mm, the gain,directivity, and the like may change, but the antenna can be put inpractical use sufficiently. The way to reduce the size of the antennaelement 10 is not limited to forming the antenna element in a meanderingpattern; the antenna element may be formed in a saw tooth wave, wave, orspiral pattern. Moreover, for the switches SWb, SWc, and SWd and theswitches SW1 and SW2, a changeover switch with a common contact that iselectrically connected to the ground conductor 14 may be used.

Furthermore, the first embodiment antenna of FIG. 1 may be modified suchthat the other end D of the antenna element 10 is electrically connecteddirectly to the ground conductor 14 without intervention of the switchSWd, as is shown in FIG. 21. Similarly, the second embodiment antenna ofFIG. 2 and the fourth embodiment antenna of FIG. 4 may be modified suchthat the other end D of the antenna element 10 is electrically connecteddirectly to the ground conductor 14 without intervention of the thirdseries resonant circuit 26 or the low-pass filter 36. In the thusmodified antenna structure, because the other end D of the antennaelement 10 is electrically connected directly to the ground conductor14, the construction becomes simpler accordingly.

FIG. 22 is an outside perspective view of a concrete example of theantenna for multiple bands of the present invention in which the otherend D of the antenna element 10 is electrically connected directly tothe ground conductor 14, shown in FIG. 21, on the assumption that theantenna is used in a mobile phone. In the example shown in FIG. 22, asubstrate 48 consists of two layers of flat circuit boards, in which arectangular ground conductor 14 is provided on the lower layer andcircuits or equivalent are arranged appropriately on the upper layer. Inone end of the upper layer of the substrate 48, corresponding to oneshort side of the ground conductor 14, the antenna element 10 formed inan meandering pattern turned around repeatedly in a direction parallelto the long sides of the rectangular ground conductor 14 is provided.The ground conductor 14 is not provided in a portion of the lower layerjust under the antenna element 10, and the antenna element 10 isprovided, separated from the ground conductor 14. One end A terminatedat a feeding point and intermediate points B and C of the antennaelement 10 are electrically connected appropriately to associatedcircuits or equivalent arranged on the upper layer and the other end Dis electrically connected to the ground conductor 14 on the lower layer.The electrical connection of the other end D to the ground conductor 14may be made by a notch made in a part of the upper layer of thesubstrate 48 or a through hole formed through the upper layer. Becausethe antenna element 10 is provided on the flat substrate 48, it is easyto form the antenna element 10. By employing the antenna element formedin the meandering pattern turned around repeatedly in a directionparallel to the long sides of the ground conductor 14, the antenna sizecan be reduced. The substrate 48 is not limited to the one consisting oftwo layers of circuit boards; it may consist of three or more layers ormay be a substrate with its front side having circuits or equivalentarranged thereon and its reverse side having the ground conductor 14provided thereon. The antenna element 10 formed in the meanderingpattern turned around repeatedly in a direction parallel to the longsides of the ground conductor 14 shown in FIG. 22 was found to have ahigh gain at a relatively high frequency band of 1800 MHz, according toan experiment.

FIG. 23 is an outside perspective view of another concrete example ofthe antenna for multiple bands of the present invention in which theother end D of the antenna element 10 is electrically connected directlyto the ground conductor 14, shown in FIG. 21, on the assumption that theantenna is used in a mobile phone. In another example of the antennashown in FIG. 23, the difference from the example shown in FIG. 22 liesin that the antenna element 10 formed in a meandering pattern turnedaround repeatedly in a direction parallel to the short sides of theground conductor 14 is provided in one end of the upper layer of thesubstrate 48, corresponding to one short side of the ground conductor14. In this another example shown in FIG. 23, an approximately middlepoint P of the antenna element is positioned, farthest separated fromthe ground conductor 14. When the antenna operates with the entirelength of the antenna element resonating with a low frequency band of800 MHz, the approximately middle point P of the antenna element 10 issubjected to the highest voltage, but its coupling is small because ofbeing farthest separated from the ground conductor 14. Thus, it ispossible to assume high antenna impedance. When the antenna operateswith a part of the antenna element 10 from the feeding point resonatingwith a relatively high frequency band without using the entire length ofthe antenna element 10, it is more likely that a point of the antennaelement where a high voltage is generated is far separated from theground conductor 14, as compared with the example shown in FIG. 22, andit is also possible to assume high antenna impedance. According to anexperiment by the inventors, a tendency was observed in which theantenna example shown in FIG. 22 has higher gain than another antennaexample shown in FIG. 23 at a high frequency band of 1800 MHz andanother antenna example shown in FIG. 23 has higher gain than theantenna example shown in FIG. 22 at a low frequency band of 800 MHz.

Then, yet another example of the antenna to which refinement from thetendency known by the above experiment is applied is shown in FIG. 24.FIG. 24 is an outside perspective view of yet another concrete exampleof the antenna for multiple bands of the present invention in which theother end D of the antenna element 10 is electrically connected directlyto the ground conductor 14, shown in FIG. 21, on the assumption that theantenna is used in a mobile phone. In this example shown in FIG. 24, thedifference from the examples shown in FIG. 22 and FIG. 23 lines in thata half part of the antenna element 10 from its one end A which iselectrically connected to the feeding point is formed in a meanderingpattern turned around repeatedly in a direction parallel to the longsides of the ground conductor 14 and the remaining half part of theantenna element up to the other end D which is electrically connected tothe ground conductor 14 is formed in a meandering pattern turned aroundrepeatedly in a direction parallel to the short sides of the groundconductor 14. At a high frequency band of 1800 MHz, the half of theantenna element 10 from its one end A, formed in a meandering patternturned around repeatedly in a direction parallel to the long sides,functions as the antenna having a high gain. At a low frequency band of800 MHz, the entire length of the antenna element functions as theantenna having a gain which is an average of the gain produced by theantenna element 10 of a meandering pattern shown in FIG. 22 and the gainproduced by the antenna element 10 of a meandering pattern shown in FIG.23. By forming the parts of the antenna element 10 enabling antennaoperation in different frequency bands in appropriate meanderingpatterns, it is possible to adjust the antenna impedance and gain.

While the antenna element 10 shown in FIG. 24 consists of the part ofthe meandering pattern turned around repeatedly in a direction parallelto the long sides of the ground conductor 14 and the part of themeandering pattern turned around repeatedly in a direction parallel tothe short sides, between these two parts, a zigzag meandering patternturned around in a direction not parallel to both the long and shortsides and a non-meandering pattern part may be inserted. The antennaelement 10 is not limited to the formation in which the half part of theantenna element 10 from its one end A which is electrically connected tothe feeding point is formed in a meandering pattern turned aroundrepeatedly in a direction parallel to the long sides and the remaininghalf part up to the other end D is formed in a meandering pattern turnedaround repeatedly in a direction parallel to the short sides. It will beappreciate that a meandering pattern part parallel to the long sides, ameandering pattern part parallel to the short sides, and anon-meandering part may appropriately constitute the antenna element.

It is not necessary to electrically connect the intermediate points B,C, and the other end D of the antenna element 10 via any one type ofelectric circuits such as the switches, series resonant circuits, andfilters to the ground conductor 14, as shown in FIGS. 1, 2, and 4. Thesepoints and the other end may be connected to the ground conductor 14 viadifferent types of electric circuits; for example, they may be connectedvia a switch, a series resonant circuit, and a filter, respectively, asis shown in FIG. 25. It will be appreciated that the resonant frequencyof a series resonant circuit consisting of a capacitor and a coil is setequal to the resonant frequency of the electrical length of the antennaelement up to the point of the connection of that circuit. Likewise, thepass frequency of a filter is set equal to the resonant frequency of theelectrical length of the antenna element up to the point of theconnection of that circuit. Thus, it is possible to electrically connectthe intermediate points B, C, and the other end D of the antenna element10 to the ground conductor 14 via any of the switches SWb, SWc, and SWd,any of the series resonant circuits 22, 24, and 26, or any of thefilters 32, 34, and 36, and there is a high degree of freedom in circuitdesign.

While, in the eighth embodiment shown in FIG. 8 and the embodimentsshown in FIGS. 22, 23, and 24, in any case, the antenna element isformed on the substrate, the antenna element may be formed on a carrierconsisting of a dielectric separate from the substrate on which circuitsor equivalent are mounted. If the dielectric is made of a highdielectric constant material such as, for example, ceramic, which isused as the carrier, the size of th antenna element can be furtherreduced. The meandering pattern of the antenna element is not limited tothat formed by angular “U” shape turns as in the foregoing embodiments;it may be formed by “V” shape or “U” shape turns or in a zigzag patternnot parallel to both the long and short sides of the ground conductor14. The meandering turns may not be always made at a constant pitch andmay be dense in one section and sparse in another section. A dimensionfrom one turn to the next turn may not be constant.

INDUSTRIAL APPLICABILITY

As described above, the antenna for multiple bands of the presentinvention is primarily configured such that one end A of the antennaelement 10 is electrically connected to the feeding point 12 and theintermediate points B, C and the other end D of the antenna element 10are electrically connected via the switches SWb, SWc, and SWd,respectively, to the ground conductor 14. The electrical length of theantenna element 10 from the one end A to the intermediate point B plusthe connection line from the point B via the switch SWb to the groundconductor 14, the electrical length of the antenna element 10 from theone end A to the intermediate point C plus the connection line from thepoint C via the switch SWc to the ground conductor 14, and theelectrical length of the antenna element 10 from the one end to theother end D plus the connection line from the other end D via the switchSWd to the ground conductor 14 are set to be capable of resonating withdifferent desired frequency bands respectively. By closing one of theswitches SWb, SWc, and SWd, one of the desired frequencies can beselected and the antenna can resonate with that frequency. Thus, theantenna employing the single antenna element 10 can operate in multiplefrequency bands and its size is easy to reduce. This antenna formultiple bands is ideal for use in a mobile phone and operation inmultiple frequency bands.

1. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point, said antenna element extending longitudinally from the feeding point to the other end thereof to obtain necessary electrical length for antenna operation, one ends of switches are connected respectively to at least one intermediate point and the other end of said antenna element, said intermediate point being a point on the longitudinally extending antenna element, the other end of one of these switches is connected to a ground conductor directly, the other ends of others of these switches are connected respectively to said ground conductor with an extension coil or a short capacitor inserted in series therebetween, different electrical lengths from said feeding point via said switches closed tip to electrical connections to said ground conductor are set to be capable of resonating different desired frequency bands respectively, and resonant frequencies with which different electrical lengths of said antenna element from said feeding point up to the connections to said switches resonate are set not to come close to one of said frequency bands with which the electrical length from said feeding point up to the connection to said ground conductor via any other switch closed resonates.
 2. The antenna for multiple bands according to claim 1, characterized in that a matching circuit is inserted between said feeding point and the one end of said antenna element and said electrical lengths including said matching circuit are set.
 3. The antenna for multiple bands according to claim 1, characterized in that a capacitor is inserted in series or capacitance is coupled between said feeding point and an intermediate point with the shortest electrical length from said feeding point.
 4. The antenna for multiple bands according to claim 1, characterized in that two parallel conductors disconnected in direct current are inserted in series so as to be inductively coupled together between said feeding point and an intermediate point with the shortest electrical length from said feeding point.
 5. The antenna for multiple bands according to claim 1, characterized in that said antenna element is formed in a meandering pattern.
 6. The antenna for multiple bands according to claim 1, characterized in that said antenna element is formed on the surfaces of a dielectric.
 7. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in an approximate rectangle and said antenna element is formed, bordering on one short side of said rectangle, separated from said ground conductor.
 8. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in an approximate rectangle on a flat substrate and said antenna element is formed on said substrate, bordering on one short side of said rectangular ground conductor, separated from said ground conductor.
 9. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in a rectangle, said antenna element is formed, bordering on one short side of the rectangle, separated from said ground conductor, and said antenna element is formed in a meandering pattern turned around repeatedly in a direction parallel to the long sides of said rectangular ground conductor.
 10. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in a rectangle, said antenna element is formed, bordering on one short side of the rectangle, separated from said ground conductor, and said antenna element is formed in a meandering pattern turned around repeatedly in a direction parallel to the short sides of said rectangular ground conductor.
 11. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in a rectangle, said antenna element is formed, bordering on one short side of the rectangle, separated from said ground conductor, one part of said antenna element is formed in a meandering pattern turned around repeatedly in a direction parallel to the long sides of said rectangular ground conductor, the remaining part of said antenna element is formed in a meandering pattern turned around repeatedly in a direction parallel to the short sides of said rectangular ground conductor.
 12. The antenna for multiple bands according to claim 1, characterized in that said ground conductor is formed in a rectangle, said antenna element is formed, bordering on one short side of the rectangle, separated from said ground conductor, a half part of said antenna element from its one end which is electrically connected to said feeding point is formed in a meandering pattern turned around repeatedly in a direction parallel to the long sides of said rectangular ground conductor, and the remaining half part of said antenna element up to the other end which is electrically connected to said ground conductor is formed in a meandering pattern turned around repeatedly in a direction parallel to the short sides of said rectangular ground conductor.
 13. The antenna for multiple bands according to claim 1, characterized in that said antenna element is formed in a meandering pattern along an imaginary circular cylinder plane and one end, the other end, and an intermediate point of said antenna element are positioned so that they are connected to and disconnected from said feeding point and the switches, the series resonant circuits, the parallel resonant circuits, or the filters.
 14. The antenna for multiple bands according to claim 1, characterized in that said antenna element is formed in a meandering pattern along an imaginary circular cylinder plane and one end, the other end, and an intermediate point of said antenna element are positioned so that they are connected to and disconnected from said feeding point and the switches, the series resonant circuits, the parallel resonant circuits, or the filters, and, in a casing in which said ground conductor, said feeding point, and the switches, the series resonant circuits, the parallel resonant circuits, or the filters are housed, said antenna element is installed in a position so as to protrude outside and to be removable.
 15. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point said antenna element extending longitudinally from the feeding point to the other end thereof to obtain necessary electrical length for antenna operation, one ends of different series resonant circuits, each comprising a capacitor and a coil, are connected respectively to at least one intermediate point and the other end of said antenna element said intermediate point being a point on the longitudinally extending antenna element, the other ends of these series resonant circuits are connected respectively to a ground conductor with an extension coil or a short inserted in series therebetween, different electrical lengths from said feeding point via said series resonant circuits up to the connections to said ground conductor are set to be capable of resonating different desired frequency bands respectively, the resonant frequency of one series resonant circuit is set equal to one of said frequency bands with which the electrical length from said feeding point up to the connection to said ground conductor via that series resonant circuit resonates, and resonant frequencies with which different electrical lengths of said antenna element from said feeding point up to the connections to said series resonant circuits resonate are set not to come close to one of said frequency bands with which the electrical length from said feeding point up to the connection to said ground conductor via any other series resonant circuit resonates.
 16. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point, one ends of different filters are connected respectively to at least one intermediate point and the other end of said antenna element, the other ends of these filters are connected respectively to a ground conductor with an extension coil or a short capacitor inserted in series therebetween, different electrical lengths from said feeding point via said filters up to the connections to said ground conductor are set to be capable of resonating different desired frequency bands respectively, each of said filters allows passage of one of said frequency bands with which the electrical length from said feeding point via the filter to the connection to said ground conductor resonates and blocks passage of one of said frequency bands with which the electrical length from the feeding point via any other filter to the connection to said ground conductor resonates, and resonant frequencies with which different electrical lengths of said antenna element from said feeding point up to the connections to said filters resonate are set not to come close to one of said frequency bands with which the electrical length from said feeding point via any other filter to the connection to said ground conductor resonates.
 17. The antenna for multiple bands according to claim 16, characterized in that said antenna element and said filters are arranged on a dielectric.
 18. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point, one ends of different parallel resonant circuits, each comprising a capacitor and a coil, are connected respectively to one intermediate point and the other end of said antenna element, the other ends of these parallel resonant circuits are connected respectively to a ground conductor with an extension coil or a short capacitor inserted in series therebetween, different electrical lengths from said feeding point via said parallel resonant circuits up to the connections to said ground conductor are set to be capable of resonating different desired frequency bands respectively, the resonant frequency of one parallel resonant circuit connected to said one intermediate point is set equal to one of said frequency bands with which the electrical length from said feeding point via said other end up to the connection to said ground conductor resonates, the resonant frequency of another parallel resonant circuit connected to said other end is set equal to another one of said frequency bands with which the electrical length from said feeding point via said one intermediate point up to the connection to said ground conductor resonates, and resonant frequencies with which different electrical lengths of said antenna element from said feeding point up to the connections to said parallel resonant circuits resonate are set not to come close to one of said frequency bands with which the electrical length from said feeding point up to the connection to said ground conductor via any other parallel resonant circuit resonates.
 19. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point, one ends of any of a switch, a series resonant circuit, comprising a capacitor and a coil, and a filter, which connected respectively to at least one intermediate point and the other end of said antenna element, the other ends of these switch, series resonant circuit, and filter are connected respectively to a ground conductor with an extension coil or a short capacitor inserted in series therebetween, different electrical lengths from said feeding point up to the electrical connections to said ground conductor are set to he capable of resonating different desired frequency bands respectively, the resonant frequency of said series resonant circuit is set equal to one of said frequency hands with which the electrical length from said feeding point up to the connection to said ground conductor via the series resonant circuit resonates, said filter allows passage of one of said frequency bands with which the electrical length from said feeding point via the filter to the connection to said ground conductor resonates and blocks passage of one of said frequency bands with which the electrical length from the feeding point to the connection to said ground conductor without the intervention of the filter resonates, and a resonant frequency with which the electrical length of said antenna element from said feeding point tip to the connection to said switch, said series resonant circuit, or said filter resonates is set not to come close to one of said frequency bands with which the electrical length from said feeding point to the connection to said ground conductor via said switch, said series resonant circuit, or said filter resonates at a different frequency from said resonant frequency.
 20. An antenna for multiple bands, characterized in that one end of an antenna element is electrically connected to a feeding point, the other end of said antenna element is electrically connected directly to the ground conductor, one end of any of a switch, a series resonant circuit, comprising a capacitor and a coil, and a filter is connected to at least one intermediate point of said antenna element, the other end of said switch, said series resonant circuit, or said filter is connected to said ground conductor with an extension coil or a short capacitor inserted in series therebetween, different electrical lengths from said feeding point up to the electrical connections to said ground conductor are set to be capable of resonating different desired frequency bands respectively, the resonant frequency of said series resonant circuit is set equal to one of said frequency bands with which the electrical length from said feeding point up to the connection to said ground conductor via the series resonant circuit resonates, said filter allows passage of one of said frequency bands with which the electrical length from said feeding point via the filter to the connection to said ground conductor resonates and blocks passage of one of said frequency bands with which the electrical length from the feeding point to the connection to said ground conductor without the intervention of the filter resonates, and a resonant frequency with which the electrical length of said antenna element from said feeding point up to the connection to said switch, said series resonant circuit, or said filter resonates is set not to come close to one of said frequency bands with which the electrical length from said feeding point to the connection to said ground conductor via said switch, said series resonant circuit, or said filter resonates at a different frequency from said resonant frequency. 